\Header{factor.scope}{Compute Allowed Changes in Adding to or Dropping from a Formula}
\alias{add.scope}{factor.scope}
\alias{drop.scope}{factor.scope}
\keyword{models}{factor.scope}
\begin{Description}\relax
\code{add.scope} and \code{drop.scope} compute those terms that can be
individually added to or dropped from a model while respecting the
hierarchy of terms.\end{Description}
\begin{Usage}
\begin{verbatim}
add.scope(terms1, terms2)
drop.scope(terms1, terms2)
factor.scope(factor, scope)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{terms1}] the terms or formula for the base model.
\item[\code{terms2}] the terms or formula for the upper (\code{add.scope}) or
lower (\code{drop.scope}) scope. If missing for \code{drop.scope} it is
taken to be the null formula, so all terms (except any intercept) are
candidates to be dropped.
\item[\code{factor}] the \code{"factor"} attribute of the terms of the base object.
\item[\code{scope}] a list with one or both components \code{drop} and
\code{add} giving the \code{"factor"} attribute of the lower and
upper scopes respectively.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{factor.scope} is not intended to be called directly by users.\end{Details}
\begin{Value}
For \code{add.scope} and \code{drop.scope} a character vector of
terms labels. For \code{factor.scope}, a list with components
\code{drop} and \code{add}, character vectors of terms labels.\end{Value}
\begin{SeeAlso}\relax
\code{\Link{add1}}, \code{\Link{drop1}},
\code{\Link{aov}}, \code{\Link{lm}}\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
add.scope( ~ a + b + c + a:b,  ~ (a + b + c)^3)
# [1] "a:c" "b:c"
drop.scope( ~ a + b + c + a:b)
# [1] "c"   "a:b"
\end{ExampleCode}
\end{Examples}

